1. Field of the Invention
The present invention relates to a method of controlling a servomotor for use as a drive source in machine tools and robots.
2. Description of the Related Art
Feed rods of a machine tool, arms of a robot or the like are driven by a servomotor which performs a velocity loop control or a position loop control. In the conventional control of the servomotor, a velocity loop gain is always set at an unvaried value allowing a certain degree of margin with respect to a gain level which may cause a mechanical resonance.
FIG. 1 is a block diagram of a servomotor position control system, by way of example, in which reference numeral 1 denotes a position control section with a position loop gain Kp; reference numeral 2 denotes a velocity control section with a integral gain k1 and a proportional gain k2; reference numeral 3 denotes a motor and mechanical system, and reference numeral 4 denotes a term integrating a velocity to derive a position.
The torque command Tc (or current command) can be derived from the procedure comprising the steps of: subtracting a position .theta. detected, for instance, by a position detector from a position command .theta.d to find a positional deviation; multiplying the positional deviation by the position loop gain Kp to find a velocity command vd; subtracting an actual velocity v detected, for instance, by a velocity detector from the velocity command vd to find a velocity deviation; adding a value, obtained by integrating the velocity deviation and then multiplying the result by the integral gain k1, to a value, obtained by multiplying the velocity deviation by the proportional gain k2, thereby obtaining a torque command Tc (current command). The servomotor is commonly driven in response to a torque command Tc (and further through a current loop control). Also, it is a common practice to control the servomotor only by means of the speed loop control without the position loop control.
In the servomotor control as described above, reductions in the positional deviation and then the velocity deviation leads to a reduction in the torque command Tc, which in turn results in a reduction in an output torque of the servomotor, thereby further preventing the positional deviation and velocity deviation from converging rapidly. Therefore, increasing the integral gain k1 and the proportional gain k2 in values not only will accelerate the convergence but improve the responsibility and the resistivity against a disturbance, thereby improving the performance of the control system. However, higher storage and proportional gains may possibly bring about a mechanical resonance as the torque command is enlarged, and lead to an adverse effect such as an overshoot or undershoot, thereby eventually causing a fluctuation in the case of greater variation in the velocity command.
In the conventional manner employing the fixed gains, the gains are restricted by the oscillation limit causing a mechanical resonance, which in turn restricts the control performance of the servomotor.